Adsorption of ammonia on graphene

We report on experimental studies of NH₃ adsorption/desorption on graphene surfaces. The study employs bottom-gated graphene field effect transistors supported on Si/SiO₂ substrates. Detection of NH₃ occurs through the shift of the source-drain resistance maximum ('Dirac peak') with the gate voltage. The observed shift of the Dirac peak toward negative gate voltages in response to NH₃ exposure is consistent with a small charge transfer (f~0.068 ± 0.004 electrons per molecule at pristine sites) from NH₃ to graphene. The desorption kinetics involves a very rapid loss of NH₃ from the top surface and a much slower removal from the bottom surface at the interface with the SiO₂ that we identify with a Fickian diffusion process.